Okay - well we actually have an example of a potentially habitable planet close to Earth that's 11.5 billion years old.
Artist's impression - see Found! Oldest Known Alien Planet That Might Support Life
So, with the universe's age estimated at 13.8 billion years (Age of the universe), that makes the universe only 2.3 billion years old when it formed.
That's Kapteyn b which is also the closest known planet orbting in its star's habitable zone, at 13 light years from the Earth. It's parent is Kapteyn's Star which is a red dwarf star, so M type.
Used to be thought that red dwarf planets are not habitable, because they are so small and dim - meaning that any planets in their habitable zone will be tidally locked - with one face of the planet constantly facing towards their "sun". However newer research taking account of the effects of clouds found that they would be more habitable than expected, with the heat distributed between the two hemispheres. The clouds both refect more light on the sunny side of the planet - and help to keep in the heat on the dark side, meaning the two hemispheres are much closer in temperature than previously thought. See: Page on iop.org
Another new development in the last few years is increasing acceptance of the idea that life could be transferred from one star system to another. For instance if one system passes through the nebula of another forming system while the planets are forming or during the late heavy bombardment.
That's been backed up by this interesting research - which seems to show that possibly life on Earth is over 9 billion years old, and couldn't have got as complex as it is in such a short time period as 4.5 billion years.
[1304.3381] Life Before Earth
So life on Earth could have begun around another star. Critics say - that you can't extrapolate back beyond the earliest known DNA based life because earlier life would use different biochemistry and probably not based on DNA either - so it is possible it just evolved far more rapidly in the first few hundred million years.
So - it is not at all conclusive, just an idea - but intriguing.
If so - life might evolve around red dwarf stars - as they are the most common type of stars and probably many have planets within the habitable zone given that we have found one already so close to Earth. Then - might then get transferred to stars like our own which are more habitable for complex lifeforms (possibly).
You also have the K-type orange dwarf stars - as you mention - which are interesting because they have far longer lifetimes than the G type stars - but also don't have tidally locked planets in the habitable zone. Might be best places for life of all - that is if a long stable sun is a good thing to have.
Habitability of orange dwarf systems
I'm not sure what is the age of the oldest orange dwarf star known - or the oldest red dwarf for that matter or oldest G type star.
But - the HR diagram is based on the mass of the star and the fuel it burns - and any of those types of stars could have formed in the early universe. And the early universe would have had a range of star sizes just as today.
So - the main question is - if those stars formed in the early solar system would they still be around today.
A light weight G type star like the Earth would have already gone through to the Red Giant phase and would be a white dwarf by now.
But larger G type stars would survive, and so would orange and red dwarfs.
I've tried to find latest figures, but seem to be different estimates in the tables shown for the same classification of stars.
Using the table from this page:
Stellar Evolution
and the table here:
The Classification of Stars
At any rate, I'm not sure if G5 type stars would still be around - but red dwarfs and orange dwarfs that formed shortly after the Big Bang should still be around.
While lighter stars would already have evolved to white dwarf stars.
The oldest stars found so far are nearly the same age as our universe.
Here is one of them,
Nearby star is almost as old as the Universe
HD 140283
It's not a main sequence star though, brighter and bluer and larger than the sun, it's a horizontal "blue branch" star. This is a state that a few of the stars heavier than the sun can enter before they become white dwarfs - they can become stable helium burning stars that last for billions of years in this "horizontal branch".
Bluish star is as old as the universe?
Horizontal branch
You'd think there must be red and orange dwarf stars nearly as old but I can't find any information about them yet.
They would however be likely to be "low metallicity stars" at that age with little more than helium, hydrogen, and lithium, so perhaps not likely to have Earth like planets.
If anyone has any more information or corrections or improvements to this do say in comments or suggest corrections!
So, with the universe's age estimated at 13.8 billion years (Age of the universe), that makes the universe only 2.3 billion years old when it formed.
That's Kapteyn b which is also the closest known planet orbting in its star's habitable zone, at 13 light years from the Earth. It's parent is Kapteyn's Star which is a red dwarf star, so M type.
Used to be thought that red dwarf planets are not habitable, because they are so small and dim - meaning that any planets in their habitable zone will be tidally locked - with one face of the planet constantly facing towards their "sun". However newer research taking account of the effects of clouds found that they would be more habitable than expected, with the heat distributed between the two hemispheres. The clouds both refect more light on the sunny side of the planet - and help to keep in the heat on the dark side, meaning the two hemispheres are much closer in temperature than previously thought. See: Page on iop.org
Another new development in the last few years is increasing acceptance of the idea that life could be transferred from one star system to another. For instance if one system passes through the nebula of another forming system while the planets are forming or during the late heavy bombardment.
That's been backed up by this interesting research - which seems to show that possibly life on Earth is over 9 billion years old, and couldn't have got as complex as it is in such a short time period as 4.5 billion years.
[1304.3381] Life Before Earth
So life on Earth could have begun around another star. Critics say - that you can't extrapolate back beyond the earliest known DNA based life because earlier life would use different biochemistry and probably not based on DNA either - so it is possible it just evolved far more rapidly in the first few hundred million years.
So - it is not at all conclusive, just an idea - but intriguing.
If so - life might evolve around red dwarf stars - as they are the most common type of stars and probably many have planets within the habitable zone given that we have found one already so close to Earth. Then - might then get transferred to stars like our own which are more habitable for complex lifeforms (possibly).
You also have the K-type orange dwarf stars - as you mention - which are interesting because they have far longer lifetimes than the G type stars - but also don't have tidally locked planets in the habitable zone. Might be best places for life of all - that is if a long stable sun is a good thing to have.
Habitability of orange dwarf systems
I'm not sure what is the age of the oldest orange dwarf star known - or the oldest red dwarf for that matter or oldest G type star.
But - the HR diagram is based on the mass of the star and the fuel it burns - and any of those types of stars could have formed in the early universe. And the early universe would have had a range of star sizes just as today.
So - the main question is - if those stars formed in the early solar system would they still be around today.
A light weight G type star like the Earth would have already gone through to the Red Giant phase and would be a white dwarf by now.
But larger G type stars would survive, and so would orange and red dwarfs.
I've tried to find latest figures, but seem to be different estimates in the tables shown for the same classification of stars.
Using the table from this page:
Stellar Evolution
and the table here:
The Classification of Stars
At any rate, I'm not sure if G5 type stars would still be around - but red dwarfs and orange dwarfs that formed shortly after the Big Bang should still be around.
While lighter stars would already have evolved to white dwarf stars.
The oldest stars found so far are nearly the same age as our universe.
Here is one of them,
Nearby star is almost as old as the Universe
HD 140283
Hertzsprung Russell diagram for M55 of stars of different masses but all around the same age - showing "Blue Branch" at top left, main sequence goes diagonally from bottom right to top left (accordiing to mass of the star) - and the "turn off" shows that the heaviest, brightest stars have already evolved off the main sequence. But the blue branch stars - though heavy - are still close to the main sequence because they have entered this steady slow helium burning phase.
It's not a main sequence star though, brighter and bluer and larger than the sun, it's a horizontal "blue branch" star. This is a state that a few of the stars heavier than the sun can enter before they become white dwarfs - they can become stable helium burning stars that last for billions of years in this "horizontal branch".
Bluish star is as old as the universe?
Horizontal branch
You'd think there must be red and orange dwarf stars nearly as old but I can't find any information about them yet.
They would however be likely to be "low metallicity stars" at that age with little more than helium, hydrogen, and lithium, so perhaps not likely to have Earth like planets.
If anyone has any more information or corrections or improvements to this do say in comments or suggest corrections!
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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